1. Field of the Invention
The present invention relates generally to wafer cleaning technology and, more particularly, to a method and apparatus for drying semiconductor wafer workpieces under manufacture.
2. Description of Related Art
One typical prior known semiconductor wafer drying apparatus is a rotary drying machine of the batch processing type, which is disclosed in, for example, Published Unexamined Japanese Patent Application ("PUJPA") No. 4-287321.
Use of the rotary wafer dryer of this type permits achievement of dry cleaned wafer surfaces by first mounting to a holder those silicon wafers rinsed by wet clean processes, rotating this wafer holder at high speeds, and then centrifugally spinning off residual water drops from the wafer surfaces.
In the case such prior art wafer dryer is employed, an unwanted pattern of traces of water flow can occur on the resultant surface of a dried wafer. This pattern is commonly referred to as the "water mark" among those skilled in the semiconductor art, which is also called the "water glass" in some cases. Such water mark is created due to the presence of silicon element of the wafer after completion of the wafer dry-clean processing, which silicon element have been dissolved in water drops and then appear through drying the wafer surface.
One example is that water marks are formed on the surface of a silicon wafer after having applied thereto the following processing including the steps of performing chemical processing for the wafer by use of dilute hydrofluoric acid, rinsing the wafer using pure water, and thereafter making the wafer dry to remove residual water drops away from its surface. Especially, in cases where the wafer has an surface configuration including "stair step"-like height differences due to fabrication of on-wafer basic elements and circuit patterns, it is difficult to completely remove water content residing at or near such step-like portions. Obviously, this would result in residence of a large number of water drops, which in turn leads to formation of many "watermark" patterns thereon. Presumably, such water marks are created in the mechanism as will be described below.
Water mark creation require three principal elements: (1) residual water content on silicon waters, (2) dissolved oxygen (DO) in water residues or solute oxygen from the environment or the both, and (3) silicon element dissolved from wafers into the residual water thereon. These elements exhibit chemical reaction within the residual water drops, as represented by: EQU Si+H.sub.2 O+O.sub.2.fwdarw.H.sub.2 SiO.sub.3,
which results in production of silicon oxides or equivalents thereto. These silicon oxides remain dissolved in the residual water drops; when water drops attempt to vapor gradually during wafer dry processes, only the silicon oxides reside on the wafer surface letting a water mark appear thereon.
The water mark thus formed might act as an electrically insulative film pattern, which leads to the cause of electrical contact deficiency or failure when fabricating at this location one or more on-wafer semiconductor basic elements such as transistors, resistors, and/or capacitors. This results in a decrease in yield of manufacturing wafers with higher-level functionalities.
In addition, at rotary wafer dry process steps, mist can generate in a way described as follows. During drying of wafers rotating at high speeds, water drops are removed away from the wafer surfaces due to the centrifugal force originated from such high-speed wafer rotation. Some or all of such spin-off water drops collide or impact against the inner walls of a drying machine used, resulting in diffusion of mist-like water in the gases held therearound. This mist can "grow" through aggregation or cohesion with other mist or dust present in the atmosphere. Resultant grown mist often adheres to the wafer surface as particles. These particles can behave to cause on-wafer IC pattern defects at later process steps. In the manufacturing process of highly integrated semiconductor devices, as on-wafer basic elements and IC patterns are becoming smaller in dimension, even "ultrafine" particles that are as small as 0.1 micrometer (.mu.m) or less in size can cause wafer defects. Additionally, as on-wafer elements and IC patterns are further scaled down in size, the relative step height of "step-like" wafer surface to step distance of the planar wafer surface increases accordingly. This in turn requires that the speed of rotation for water spin-off removal be further increased in order to more successfully eliminate creation of water marks on the wafer surfaces. But there is a trade-off: Making the rotation speed higher can inhibit residence of water content on wafer surfaces, but making it too high can also increase production of mist per se. This will become a serious bar to further development of ultralarge-scale integration (ULSI) technologies in near future.
Another approach to wafer drying as the alternative to rotary drying schemes is to employ low-pressure drying techniques. A typical low-pressure drying method includes the steps of loading into a wafer drying chamber a set of clean wafers rinsed with pure water, and then vacuum-evacuating the interior of this chamber down at low pressures permitting vaporization of residual water content on the wafer surfaces to thereby make the wafer dry in the chamber.
Unfortunately, this approach suffers from incomplete avoidability of particles residing on the wafer surfaces processed. More specifically, when evacuating inside of the chamber for establishment of pressure reduction therein, the internal temperature of the chamber decreases accordingly. When the temperature dropping down to the saturated vapor temperature of residual water content, the residual water begin to boil suddenly. When this happens, secondary boiling or vaporization can simultaneously take place also from the inside of such water residues letting part of water be lifted off in the form of mist during boiling. This mist may grow by aggregating with mists or dusts inherent in the chamber interior. These large resultant mists may adhere to the wafer surfaces and then may act as harmful particles, which might serve to cause on-wafer pattern defects at later process steps. Also note that upon vaporization of water content, the wafer surface temperature decreases due to dispersion of the vaporization heat. When the wafer temperature is at or near a specific level equivalent to the solidifying or freezing point of water (i.e. 0.degree. C.), water should experience both ebullition and solidification at a time. Solidified water is released by liberation to further accelerate unwanted production of mists.
As stated above, prior art wafer drying schemes are associated with problems including creation of water marks on dried wafer surfaces and adhesion of harmful particles and contamination thereto. These problems raise a serious bar to semiconductor device manufacturing processes.